1. Field of the Invention
The present invention relates to soft magnetic alloy thin films for use in, for example, thin-film inductors and transformers, and also to magnetic devices incorporating the thin films.
2. Description of the Related Art
Soft magnetic alloys are used to produce, for example, magnetic head cores, thin-film inductors, transformers and choke coils. For these devices to operate adequately, the soft magnetic alloys must exhibit magnetic properties such as high saturation magnetic flux density, high magnetic permeability, low coercive force and the ability to be formed into a thin film.
Various alloy compositions have been studied to identify alloys which can be used to produce suitable soft magnetic alloys. Conventionally, crystalline alloys such as Sendust (Fe-Al-Si alloy), permalloy (Fe-Ni alloy), and silicon steel (Fe-Si alloy) have been employed as soft magnetic alloys. More recently, Fe-based and Co-based amorphous alloys have also been used.
Although the conventional alloys are adequate for some applications, the recent trend toward smaller components and higher operating frequencies necessitates the development of new materials having even better soft magnetic properties for use as thin-film inductors, transformers, and choke coils.
Although Sendust has adequate soft magnetic properties for some applications, it is limited in that its saturation magnetic flux density is about 1.1 tesla (T). Permalloy also can exhibit adequate soft magnetic properties, but is limited in that its saturation magnetic flux density is as low as 0.8 T. Silicon steel has high saturation magnetic flux density, but exhibit poor soft magnetic properties.
Another crystalline soft magnetic alloy thin film is disclosed in U.S. Pat. No. 5,117,321. This crystalline soft magnetic alloy comprises an Fe-B-N system, where B represents at least one element selected from the group consisting of Zr, Hf, Ti, Nb, Ta, V, Mo, and W, and its compositional ranges in atomic percent are 0&lt;B&lt;20 and 0&lt;N&lt;22, wherein when B.ltoreq.7.5, then N&gt;5, and when N.ltoreq.5, then B&gt;7.5.
The soft magnetic alloy thin film disclosed in U.S. Pat. No. 5,117,321 is obtained by applying heat treatment after film formation to change the amorphous phase to a crystalline phase. The resulting crystalline soft magnetic alloy thin film has a high magnetic permeability at low frequencies (below 20 MHz), a high saturation magnetic flux density, and low coercive force. The composition can be adjusted to eliminate magnetostriction. However, the imaginary component of the magnetic permeability of this thin film becomes larger than the real component at frequencies higher than about 20 MHz, as discussed below and shown in FIG. 16 of this application. When the imaginary component exceeds the real component, the total magnetic permeability is substantially reduced. Therefore, the crystalline alloy disclosed in U.S. Pat. No. 5,117,321 is inadequate for applications above 20 MHz.
On the other hand, Co-based amorphous alloys have adequate soft magnetic properties, but exhibit poor saturation magnetic flux density (1.0 T). Although Fe-based amorphous alloys have a saturation magnetic flux density of 1.5 T or more, they also exhibit poor soft magnetic properties. In addition, Co-based and Fe-based amorphous alloys do not have sufficient thermal-stability.
As described above, it has been difficult for conventional materials to have both high saturation magnetic flux density and adequate soft magnetic properties for use in high frequency (i.e., above 20 MHz) applications.